I saw the following statement on the Cisco site about the 9124.
Each port group consisting of 4 ports has a pool of 64 buffer credits, with a default of 16 buffer credits per port. When extended distances are required, up to 61 buffer credits can be allocated to a single port within the port group. This extensibility is available without additional licensing.
This leads me to how many B2B credits are needed for 10 Km when using 2 GB ISL's? If I wanted to run 2 x 2 GB ISL as a port channel, I suppose they have to be in different port groups especially if I had to go for 20 or 30 Km. If I make a port an TE port, can the other ports in the port group be used? I know that 2nd Gen cards on the MDS 95xx series allow this but I don't know about the 9124.
yes, you can still use the other ports in the same port group even if you configure one as TE (that was a limitation of 9120 and 9140 only, first generation of products; 9124 is based on a new Switch on Chip architecture).
If you are running the links at 2Gbps, and you want to reach 20-30 km you will need from 20 to 30 buffer to buffer, having more than half left to be shared by remaining ports in the port group.